The entire disclosure of Japanese Patent Applications No. 2000-192514 filed on Jun. 27, 2000 and No. 2001-154887 filed on May 24, 2001 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to an apparatus for refining sodium (hereinafter referred to as a sodium refining apparatus), the sodium containing impurities such as oxides and hydroxides, and to a system for refining sodium ago (hereinafter referred to as a sodium refining system).
2. Description of the Related Art
Sodium is employed as a coolant or like material in facilities such as nuclear power plants, and impurities such as oxides and hydroxides possibly migrate into sodium during its use.
Conventionally, some impurities are removed through a technique such as cold trapping, in which sodium is cooled and impurities are trapped by use of metallic material such as zirconium (Zr).
Although suitable for removing impurities such as oxygen and hydrogen, cold trapping is not suitable for removing impurities such as oxides and hydroxides.
Thus, there has previously been proposed a sodium refining apparatus attaining high purity on the basis of a technique of an alkali metal thermo-electric converter (AMTEC) (Japanese Patent Application Laid Open (kokai) No. 6-172883).
FIG. 11 (PRIOR ART) shows a schematic representation of the apparatus disclosed in the above publication.
In FIG. 11 (PRIOR ART), xcex2xe2x80x3-alumina (hereinafter referred to simply as xe2x80x9cxcex2-aluminaxe2x80x9d) is employed as a solid electrolyte. A heating chamber 03 and a condensation chamber 04 are provided, along with a xcex2alumina-made separator 01 disposed therebetween. In the condensation chamber 04, a porous electrode 02 is formed on the separator 01. A lead connecting the porous electrode 02 with impurity-containing sodium 06 contained in the heating chamber 03 is electrically connected to a resistor 010, a heater 07 provided in the heating chamber 03, or cooling means 013 for cooling a cooling section 012 of the condensation chamber 04.
In such an apparatus, sodium is heated to 900-1,300 K, to thereby form sodium cations. The difference in vapor pressure between the heating chamber and the condensation chamber urges the thus-formed sodium cations to transfer through the solid electrolyte, and the cations reach the surface (facing the cooling section of the condensation chamber) of the solid electrolyte. The released electrons are supplied, via a lead connecting the porous electrode with sodium contained in the heating chamber, to the interface between the porous electrode and the solid electrolyte, where the electrons are recombined with the sodium ions which have been supplied through the solid electrolyte. The thus-formed electrically neutral sodium vaporizes at the surface of the electrolyte and is condensed in the cooling section, to thereby yield pure sodium.
However, during operation of the aforementioned prior art refining apparatus, sodium contained in the heating chamber 03 must be heated to at least 900 K (623xc2x0 C.), and therefore, deterioration of xcex2-alumina is accelerated, resulting in poor durability.
In addition, the differences in temperature and vapor pressure of the sodium chamber must be maintained constant throughout the refining process, and the porous electrode must be attached directly to the surface of the electrolyte. Thus, configuration and operation of such an apparatus require increased costs.
Although xcex2-alumina is suitable for refining sodium; i.e., removing impurities such as oxides and hydroxides, efficient removal of oxygen cannot be attained. Thus, when the sodium refined by use of xcex2-alumina is used for a long period of time, corrosion of piping in the apparatus may occur. In order to prevent this problematic corrosion, cold trap means must be added, but such additional means inevitably increases the size of the refining apparatus.
In view of the foregoing, the present inventors have carried out extensive studies to solve the problems. Accordingly, an object of the present invention is to provide a sodium refining apparatus of simple structure which is free from the problem of deterioration of solid electrolyte. Another object of the invention is to provide a sodium refining system including the refining apparatus.
In one aspect of the present invention, there is provided an apparatus for refining sodium, in which impurities contained in sodium are removed by a solid electrolyte having sodium ion conductivity, the apparatus comprising:
a bottom-closed casing made of a solid electrolyte and for containing impurity-containing sodium or a small amount of highly pure sodium;
an outer casing for accommodating said bottom-closed casing and for containing, outside said bottom-closed casing, a small amount of highly pure sodium when said bottom-closed casing contains impurity-containing sodium, and impurity-containing sodium when said bottom-closed casing contains highly pure sodium;
a first electrode to be inserted in the impurity-containing sodium or in the highly pure sodium;
a second electrode to be inserted in the highly pure sodium when the first electrode is inserted in the impurity-containing sodium, or in the impurity-containing sodium when the first electrode is inserted in the highly pure sodium; and
a power source for applying DC voltage to the electrodes;
wherein
the impurity-containing sodium and the highly pure sodium are in electrical contact with each other via the solid electrolyte;
and when the DC voltage is applied, the impurity-containing sodium is positively charged and the highly pure sodium is negatively charged, to thereby ionize sodium contained in the impurity-containing sodium; and
the thus-formed sodium cations are caused to pass through the solid electrolyte and, subsequently, are combined with electrons at the surface of the solid electrolyte, to thereby yield refined sodium.
Preferably, the liquid-surface level of the bottom-closed casing formed of solid electrolyte and that of the outer casing are adjusted to be approximately equal to each other.
Preferably, the solid electrolyte is formed of xcex2-alumina.
Preferably, the electrodes are formed of a material which is highly anti-corrosive against sodium, such as molybdenum (Mo), tungsten (W), or stainless steel.
Preferably, in the apparatus, sodium is refined at 200-500xc2x0 C.
In another aspect of the present invention, there is provided a system for refining sodium, the system comprising the aforementioned apparatus for refining sodium; supply means for supplying impurity-containing sodium into the outer casing of the apparatus for refining sodium; and sodium-recovery means for recovering sodium refined by means of the apparatus for refining sodium.
Preferably, the system further includes oxygen-removal means for removing oxygen contained in refined sodium.
Preferably, in the system, refined sodium is supplied from the sodium-recovery means to a reactor; the supplied sodium is used in the reactor; and, subsequently, the resultant impurity-containing sodium is supplied again to the supply means for supplying impurity-containing sodium.
Preferably, in the system, the impurity-containing sodium is a coolant used in a fast-breed reactor.